Conventional organic brake linings are made up of a mixture of organic binders, asbestos fibers and property modifiers. The organic binders usually include some resin that solidifies upon heating. The asbestos fibers, which strengthen the lining, remain relatively stable when subjected to thermal changes up to 800.degree. F. However, asbestos deteriorates at higher temperatures due to loss of water, of crystallization and frictional wear.
Modifiers, varied in type and content, must also be added to provide a desired level of effectiveness with respect to wear resistance, fade resistance and noise.
However, when organic brake linings are exposed to high temperature such as that created by rapidly repeated frictional engagement over a period of time, the thermally unstable elements of the lining mixture undergo a chemical or structural change. Asbestos dehydrates and is eventually transformed into forsterite or olivine. Inorganic modifiers such as carbonates and sulfates decompose and the resin binder loses its stability. When organic elements in the brake lining decompose upon being subject to a high temperature environment, that portion of the decomposed element remaining upon termination of the high temperature is carbon. This created carbon is on the surface of the brake lining and has no adverse effect on the coefficient of friction of brake lining since it is not an integral bound part thereof. The extent of breakdown or wear of this type of organic lining is directly dependent upon the time and length of exposure to an elevated temperature.
Correspondingly in such an elevated temperature the coefficient of friction is generally reduced due to thermal degration of the organic component. This lowering of the coefficient of the material proportionally reduces the effectiveness in frictionally stopping.
Later as disclosed in U.S. Pat. No. 3,434,998 assigned to the common assignee of this application and incorporated herein by reference, the asbestos fiber of the organic brake lining was replaced with a metal fiber and fillings and graphite powder used as a friction modifier. The metal fiber and fillings can undergo reversible transformation from a solid to a liquid when subjected to a predetermined temperature about 1500.degree. F. The graphite powder is thermally stable in this temperature range and as a result a structurally stable lining material can be produced. However, graphite has a low coefficient of friction necessitating the need for a surface area wherein the combined coefficient of friction graphite and metal fiber produces an overall coefficient of friction for the lining sufficient to bring a vehicle to a stop within a predetermined distance from 60 miles per hour. Normally, the graphite content in the lining material is greater than the metal fiber and filling. Increasing the graphite to metal fiber and filling ratio results in an overall lower coefficient of friction and performance; however, a material which wears better and internally absorbs the noise created by frictional engagement is produced.
Conversely, decreasing the graphite to metal fiber and filling ratio results in a higher coefficient of friction and performance. However, wear and noise is substantially decreased.